The present invention relates to thiadiazole derivatives useful as accelerators and/or curing agents for halogenated polymers in rubber vulcanization processes, and to halogenated polymer compositions containing the thiadiazole derivatives, as well as a method of preparing the same.
Vulcanizable rubber compositions present certain inherent problems in terms of handling and storage. For example, prior to the curing, the uncured rubber may often degrade during storage due to hydrolytic instability of the additives contained therein. As a result the cure reproducibility from batch to batch in the vulcanization process can often vary. However, batch-to-batch cure reproducibility is an important parameter of quality control.
It is known that halogen-containing polymers may be compounded with curing agents, accelerators and other compounds in order to prepare vulcanizable rubber compositions which are useful in a variety of applications. A description of curing agents and accelerators, as well as other components of natural and synthetic rubbers can be found in Kirk-Othmer""s Encyclopedia of Chemical Technology, John Wiley and Sons, 4th Edition, at pages 460-481.
Despite the availability of curing agents and/or accelerators for halogenated polymers, there is a continuing need for curing agents and/or accelerators that allow for good bin storage characteristics and improved batch-to-batch cure reproducibility.
Accordingly, it is an object of the present invention to compounds useful as curing agents and/or accelerators for rubber vulcanization processes which provide good bin storage properties for uncured rubber and improved consistency in cure reproducibility.
It is yet another object of the present invention to provide curable rubber compositions which exhibit good bin storage properties and improved consistency in cure reproducibility and methods of preparing the cured rubber compositions.
In accordance with the present invention, an additive is provided including a dithiocarbamyl-1,3,4,-thiadiazole derivative having formula (I), or an isomer thereof: 
where R1 and R2 are independently a radical being either an alkyl, a cycloalkyl, an alkenyl, an aryl, an arylalkyl, or an alkylaryl, or R1 and R2 together form a 3- to 7-membered cyclic ring structure; and X is (i) hydrogen, (ii) a dithiocarbamyl radical having formula (II): 
where R3 and R4 are independently a radical being either an alkyl, a cycloalkyl, an alkenyl, an aryl, an arylalkyl, or an alkylaryl, or R3 and R4 together form 3- to 7-membered cyclic ring structure, or (iii) a mixture thereof. In a preferred embodiment, X is hydrogen, and R1 and R2 are independently a radical being either an ethyl, an isopropyl, a butyl, or an isobutyl, or R1 and R2 together form a 6-membered cyclic ring structure, with a piperidyl radical being preferred.
In another embodiment the present invention provides an additive including a dithiocarbamyl-bis-1,3,4,-thiadiazole derivative having formula (III), or an isomer thereof: 
where X is hydrogcn or a dithiocarbamyl radical having formula (II) 
where R3 and R4 are independently a radical being either an alkyl, a cycloalkyl, an alkenyl, an aryl, an arylalkyl, or an alkylaryl, or R3 and R4 together form 3- to 7-membered cyclic ring structure. In a preferred embodiment, X is hydrogen.
The present invention also provides a curable polymer composition including at least one halogenated polymer and at least one of the above-described additives of the present invention. A method is also provided for preparing a cured polymer composition including at least one halogenated polymer and at least one of the additives of the invention.
The additives of the present invention are particularly useful as curing agents and/or accelerators for halogen-containing polymer compositions, and provide good bin storage characteristics for the uncured polymer composition and improved cure reproducibility. These and other advantages of the present invention will be more readily apparent from the detailed description set forth below.
It has now been discovered that certain hydrocarbyl dithiocarbamyl-1,3,4-thiadiazole derivatives having a disulfide linkage are useful as curing agents and accelerators in halogen-containing polymer compositions. The thiadiazole derivatives of the present invention have been found to provide good bin storage characteristics and improved cure reproducibility when used as additives in halogen-containing polymer compositions.
In one embodiment an additive is provided that includes a dithiocarbamyl-1,3,4,-thiadiazole derivative having formula (I): 
where R1 and R2 are independently a radical being either an alkyl, a cycloalkyl, an alkenyl, an aryl, an arylalkyl, or an alkylaryl, or R1 and R2 together form a 3- to 7-membered cyclic ring structure; and X is (i) hydrogen, (ii) a dithiocarbamyl (i.e., DTC) radical having formula (II): 
where R3 and R4 are independently a radical being either an alkyl, a cycloalkyl, an alkenyl, an aryl, an arylalkyl, or an alkylaryl, or R3 and R4 together form substituted or unsubstituted 3- to 7-membered cyclic ring structure, or (iii) a mixture thereof. Preferably, X is hydrogen while R1 and R2 are independently a C1 to C5 alkyl radical such as an ethyl, an isopropyl, a butyl, or an isobutyl, or R1 and R2 together form a substituted or unsubstituted 6-membered cyclic ring structure (e.g., a piperidyl radical).
In another embodiment the present invention provides an additive that includes a dithiocarbamyl-bis-1,3,4,-thiadiazole derivative having formula (III): 
where X is (i) hydrogen, (ii) a dithiocarbamyl radical having formula (II): 
where R3 and R4 are independently a radical being either an alkyl, a cycloalkyl, an alkenyl, an aryl, an arylalkyl, or an alkylaryl, or R3 and R4 together form 3- to 7-membered cyclic ring structure, or (iii) a mixture thereof. In a preferred embodiment, X is hydrogen.
The derivatives of formulas (I) and (III) are synthesized following techniques known in the art. For example, compounds having formula (I) can be synthesized by reacting 2,5-dimercapto-1,3,4,-thiadiazole (i.e., DMTD) with a dithiocarbamic acid in which the amine moiety is a tertiary amine having substituents R1 and R2, or R3 and R4. The reaction is carried out in the presence of an oxidizing agent (e.g., hydrogen peroxide). As known in the art, dithiocarbamic acid is not readily isolatable and thus needs to be formed in situ to provide the starting material. The dithiocarbamic acid intermediate is synthesized in situ by reacting carbon disulfide with the appropriate secondary amine (e.g., dibutyl amine to form dibutyl dithiocarbamic acid). Likewise, compounds of formula (III) where X is hydrogen can be synthesized by first forming in situ bis-(1,4-piperazine dithiocarbamic acid) from homopiperazine and carbon disulfide. The dithiocarbamic acid intermediate is then reacted with 2,5-dimercapto-1,3,4-thiadiazole in the presence of an oxidizing agent. The reaction conditions (e.g., temperature and time) are variable and can be easily modified by one of ordinary skill in the art following the teachings set forth herein.
While not wishing to be limited by theory, those skilled in the art will recognize that the additives of the invention may additionally contain positional isomers of the derivatives having formulas (I) and (III) due to tautomerization or other similar rearrangement of the substituents on the DMTD moiety. In accordance with the invention, reference to xe2x80x9can isomer thereofxe2x80x9d means positional isomers. Positional isomers of formulas (I) and (III) are also useful as curing agents and accelerators. Positional isomers of formula (I) would have the following structures: 
Likewise, similar rearrangements for compounds having formula (III) are also expected.
The above-described additives of the invention, alone or in combination, are admixed with a major amount of at least one halogenated polymer to provide a curable (i.e., vulcanizable) polymer composition. The polymer composition is then cured following conventional techniques known in the art to produce a cured (i.e., vulcanized) polymer composition.
In accordance with the present invention, any saturated or unsaturated halogen-containing (i.e., halogenated) polymer may be used. Preferably, the polymer contains at least one percent by weight halogen with about five percent being more preferred. The halogen content in the polymer may range up to 40 percent based on the weight of the polymer. Preferably, the halogen-containing polymers is an elastomer. In another preferred embodiment, the halogen-containing polymer is a chlorine-containing (i.e., chlorinated) polymer. Representative examples of chlorine-containing polymers to be used in accordance with the present invention include, but are not limited to, homopolymers of epichlorohydrin, copolymers of epichlorohydrin and ethylene oxide or propylene oxide, polychloroprene, chlorinated polyolefins, chlorosulfonated polyolefin, polychloroalkylacrylates and chlorobutyl rubber. These polymers are well known in the art and are available commercially from variety of sources. One particularly preferred chlorinated polymer is chlorinated polyethylene xe2x80x9cCPExe2x80x9d which is commercially available from DuPont Dow under the tradename Tyrin(copyright).
The halogen-containing polymers may be blended with non-halogen containing polymers as along as a sufficient halogen content is provided in the polymer composition to effect crosslinking. The blends can include, but are not limited to, natural ruber, polybutadiene, polyolefins, copolymers of butadiene with styrene (SBR) or acrylonitrile (NBR), copolymers of ethylene-propylene-diene (EPDM), butyl rubber and the like. Such blends may contain from about 10 to about 90% by weight of each type of polymer. In a more preferred embodiment, the blends contain the halogenated polymer at levels from about 25 to 75% by weight with respect to the total weight of the polymer blend.
The additives of the invention may be incorporated into the polymer composition in their pure form or they may be mixed with one or more liquid diluents. They also may be adsorbed onto the surface of a finely divided, inert carrier to provide a powdered product. When the additives of the invention are mixed with a liquid diluent or finely divided carrier, the additive may range from 15 to 85 percent by weight of the composition with the remainder being the diluent, carrier or a combination thereof. Preferably, the additives of the invention are mixed in a ratio ranging from 30 to 70 percent by weight.
The suitable diluents, among others, include aromatic, naphthenic and paraffinic hydrocarbon oil, polyglycols and glycols, alkyl esters of dibasic acids, e.g., dioctyl phthalate, dioctyl sebacate, dioctyl adipate, diisodecyl glutarate, dioctyl azolate, alkyl sulfides, fatty acid esters, e.g., butyl oleate, butyl stearate, octyl epoxy tallate, trioctyl trimellitate, polyester plasticizers, e.g., polymeric di(butoxy-ethoxy-ethyl)adipate, polymers of bis(ethyleneoxy)methane with disulfide linkages; petroleum sulfonates, alkyl trimellitates; and polymeric esters.
The suitable finely divided carrier materials include carbon black, metal oxides, such as aluminum oxide, alumina, silica, mineral fillers, such as clay, talc and betonite, aluminosilicate, zeolites, calcium silicate and similar carriers. Preferred carriers have a surface area of from about 75 to about 300 m2/gm. A particularly preferred carrier is amorphous silica available from Pittsburgh Plate Glass Company under the tradename HISIL(copyright)233 and HISIL(copyright) ABS.
The amount of the additive effective to cure the chlorinated polymer will vary as a function of the halogen content in the halogenated polymer. Generally, the additives are employed in the range from about 0.1 to about 10.0 parts by weight per 100 parts by weight of halogenated polymer present in the curable composition. More preferably, the additives of the present invention are present in the amount from about 0.5 to about 5.0 parts by weight per 100 parts by weight of the halogenated polymer. If a diluent or a carrier material is added to the curable polymer composition, higher levels of the additive may be required.
Additional accelerators of the aliphatic or aromatic amine type can also be used if the halogenated polymer employed for production of vulcanized rubber is relatively unreactive. The suitable accelerators, among others, include the reaction product of butyraldehyde and aniline (available commercially under the tradename VANAX(copyright) RTM 808 from R. T. Vanderbilt Company, Inc.), fatty amines, sulfonamides such as N-cyclohexyl-2-benzothiazolesulfenamide (available commercially under the tradename DURAX(copyright) from R. T. Vanderbilt Company, Inc.) and quaternary ammonium salts, such as tetrabutylammonium bromide and tetraethylammonium chloride. A listing of additional accelerators to be utilized in accordance with the present invention is set forth in xe2x80x9cRubber Chemicals,xe2x80x9d J. Van Alphen, pages 1-46 (1973), which is incorporated herein by reference.
For curing blends of halogenated and non-halogenated polymers, sulfur or other well known sulfur-containing curatives for unsaturated elastomers may be included in the composition. Examples of such compounds include, but are not limited to, sulfur, benzothiazyl disulfide, N-oxydiethylene benzothiazole-2-sulfonamide, 2-mercaptobenzo-thiazole, alkyl phenol disulfides, tetraalkylthiuram disulfide and monosulfide having normal or branched chain alkyl groups, m-phenylene-bismaleimide and N,Nxe2x80x2-diarylguanidines.
Other additives, which may be desirable to effect crosslinking along with the derivatives of the present invention, include basic metal oxides, metal hydroxides and metal salts of carboxylic acids. The typical additives include zinc oxide, magnesium oxide, zinc stearate and sodium acetate. The magnesium oxide may be synthetic or a natural magnesite mineral. The magnesite may be calcined or treated by other similar processes to yield a predominantly magnesium oxide product.
In addition to the curatives, the polymer compositions of the invention may also include antioxidants, for example, octylated diphenylamine, diphenyl-p-phenylenediamine and styrenated phenol type antioxidants. Likewise, the polymer compositions of the invention may include antidegradants, antiozonants, antiflexcracking agents, heat stabilizers and metal poison-inhibitors, which are well known in the art.
The curable compositions may be prepared and blended using any suitable mixing device such as a two-roll mill, an internal mixer (Brabender Plasticorder), a Banbury Mixer, a kneader or a similar mixing device. The processing and vulcanization techniques are well known in the art.
The following non-limiting examples are given to further illustrate the additives of the invention and their use in curable polymer compositions. All percentages and parts are based on weight unless otherwise indicated.